Lepidocrocite association with goethite

Where lepidocrocite forms from Fe salt solutions it is often associated with goethite. It has been observed in partially hydrolysed Fe nitrate solutions at low pH (Murphy et al. 1976c) (Fig. 13.5) and during very slow hydrolysis at pH 7 (see Fig. 13.3) (Schwertmann Cornell, 2000) suggesting that it can form directly from low concentrations of low-molecular weight precursors. 

Fig. 16.5 Electron micrographs of an association of lepidocrocite (Lp) with goethite (Gt) from a redoximorphic soil, Natal, South Africa (courtesy P. Self).

Schwertmann, U. Fitzpatrick, R.W. (1977) Occurrence of lepidocrocite and its association with goethite in Natal soils. Soil Sci. Soc. 

Carlson, L. Schwertmann, U. (1981) Natural ferrihydrites in surface deposits from Finland and their association with silica. Geochim. Cosmochim. Acta 45 421-429 Carlson, L. Schwertmann, U. (1987) Iron and manganese oxides in Finnish ground water treatment plants. Wat. Res. 21 165-170 Carlson, L. Schwertmann, U. (1990) The effect of CO2 and oxidation rate on the formation of goethite versus lepidocrocite from an Fe(II) system at pH 6 and 7. Clay Min. 25 65-71... 

In another report. Hansel et al. (2002) studied Fe plaque associated with the rhizosphere of Typha latifolia and Phalaris arundinacea from a mine waste-contaminated site that was high in As. For both plants, the Fe and As concentrations associated with roots were approximately 10-fold concentrated on roots relative to their abundances in the bulk soil. XRF revealed that Fe plaque consisted primarily of ferrihydrite but also had appreciable levels of goethite and siderite T. latifolia also had a significant amount of lepidocrocite. Their analysis suggested that As was sequestered fairly homogeneously within the Fe plaque. A XANES analysis indicated that approximately 80% of the As was As(V) and 20% As(III). Blute et al. (2004) reported similar ratios for As(V) and As(III)... 

It should be noted that minerals, and oxide minerals in particular, have different types of OH groups, depending on the coordination of the O atoms, as revealed by spectroscopic studies. Goethite (a-FeOOH) has four types of surface hydroxyls whose reactivities are a function of the coordination environment of the O in the FeOH group (Sposito 1984 Sparks 2002). The FeOH groups are A-, B-, or C-type sites, depending on whether the O is coordinated with 1,3, or 2 adjacent Fe(III) ions. The fourth type of site is a Lewis acid-type site, which results from chemisorption of a water molecule on a bare Fe(III) ion. Only A-type sites are basic (can bind H+), and, on the contrary, A-type and Lewis acid sites can release a proton. The B- and C-type sites are considered unreactive. Thus, A-type sites can be either a proton acceptor or a proton donor (i.e., they are amphoteric). Other spectroscopic studies have shown that boehmite (y-AlOOH) and lepidocrocite (y-FeOOH) have two types of OH, presumably associated with different crystal faces (Lewis and Farmer 1986). 

This method of analysis is particularly valuable when chemical methods are inadequate or inapplicable. For instance, for complex mixtures where the different elements or ions may be associated in many different ways, all compatible with the analytical figures or for mixtures of polymorphous forms of the same substance, such as the three crystalline forms of CaC03 (calcite, aragonite, and vaterite) or the three crystalline forms of FeO(OH) (goethite, lepidocrocite, and P FeO(OH)—see Bunn, 1941)—mixtures for which chemical analytical methods are irrelevant. 

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